The present invention pertains to load handling equipment and, more particularly, to an improved turn conveyor system.
Throughout industry and agriculture, conveyors are used to move and process products and materials.
One type of conveyor used for these applications is called a "turn conveyor". Turn conveyors typically consist of an infeed section, an arced or radius section, and a discharge section.
The carrying surface or "belting" for these conveyors is of two main types, fixed radius and variable radius.
Variable radius belting is constructed in a fashion that enables it to move uninterruptedly through straight sections and turn sections, eliminating product transfer points that would otherwise disturb the product flow. This type of belting can, within certain limits, conform or "side flex" to different inside radii, enabling a much greater variety of layout configurations.
Fixed radius belts conform to an arced section only, with no continuity between adjoining straight sections, necessitating product transfers onto and off of the turn section.
Both fixed radius and variable radius belts are constructed of metal and plastic.
There are a number of different belt designs employed to similar effect. There are, however, technical limitations and problems common to all these belts that greatly reduce the conveyor design possibilities and limit effective loading on these belts. These problems are all manifest as destructive levels of belt tension, and the proximate cause is friction. This friction has three sources:
1. Contact between the "inside" or minor radius edge of a radius conveyor belt and the conveyor structure that defines this radius.
2. Contact between the "outer" or major radius edge of a radius conveyor belt and any device that are used to hold it down; that is, prevent the belting from "lifting" or "flipping up" from excessive tension as it moves through the radius portion of a conveyor. (These "hold down" devices are sometimes used on the minor radius edge as well).
3. Friction between the "bottom" or underside of the conveyor belt and the surface on which it rides.
Currently, typical construction for turn conveyors has a metal frame supporting a metal or plastic conveyor belt. To help reduce both wear and friction, plastic stripping is applied at the appropriate contact points on the frame. This "wear stripping", is usually ultra high molecular weight (U.H.M.W.) polyethylene or an "engineering" grade nylon.
Typically, the best coefficient of friction that can be expected is 0.2 and that increses drastically under usual operating conditions when dirt and production debris enter between the belt and the frame.
At present, manufacturers of radius belting construct the belt edges of the same material as the belt body. Typically, this means stainless steel belting with stainless steel edges, and polyethylene or polypropylene belts with edges of these respective materials. This is true for both fixed and variable radius belting.
The only known exception to this is a fixed radius belt manufactured by Pulver Systems of Chicago Ridge, Ill. This belt, known as a "Meshlok" turn belt, is of a rod type construction, with the outer edge of the belt being of a greater pitch than the inner, thus creating and describing the radius which the belt conforms. In this belt, journals are turned in the ends of each rod to accept connection links of flat steel that have been appropriately perforated. A shoulder is left on each rod end. This shoulder is of the same diameter as the main body of the rod and approximately 3/16' long. Nylon caps "snap" over the rod ends to retain the connection links and act as a bearing surface on the inside. On the outside, metal spring clips are used for retention, apparently no consideration having been given to the use of bearing grade materials on the outside edge.
A number of problems arise with this design:
1. Rod caps frequently pop off during conveyor use causing product contamination and exposing metal rods to what is typically a metal wear surface, thereby generating metal contaminants.
2. Nylon, the cap material has a coefficient of friction of 0.2 which is approximately the same as most other belt edge materials coacting with their respective wear surfaces, consequently producing no significant friction reduction.
3. The rod caps have severly limited bearing surface, and thus, suffer a high bearing pressure, which greatly increases the chances of galling of the bearing surfaces and deformation of the plastic rod cap.
4. With the type of connection link used, the belt rods can "articulate", allowing a single rod to move away from the plane or "track" of belt movement under heavy loads or extremely high speed or when a rod is bent This phenomenon tends to exacerbate problems 1 and 3.